(1) Field of the Invention
The present invention relates to a field programmable device, more particularly to a field programmable device suitable for tests for detecting defective operations thereof. The field programmable devices to which the present invention relates include, for example, a ROM (read-only memory), a PROM (programmable read-only memory) and a FPLA (field programmable logic array).
(2) Description of the Prior Art
A conventional field programmable device comprises a memory cell array. The memory cell array comprises memory cells connected between bit lines and word lines at intersections of the bit lines and the word lines. A write-in operation is effected in the memory cells by a user, after the field programmable device is shipped from the manufacturer to the user, in accordance with the program or data desired by the user. Therefore, when the field programmable device is shipped to the user, the memory cells are in their original state with no writing-in of information. However, it is necessary to test the field programmable device for satisfactory operation before shipment. For this purpose, there are provided test memory cells selectively placed in a conductive or nonconductive state through application of predetermined data signals. The test memory cells are connected between test bit lines and word lines at the intersections thereof. The test memory cells may also be connected between bit lines and test word lines. These test bit lines and test word lines are placed adjacent to the group of bit lines and word lines.
In a prior art test system for testing field programmable devices disclosed in U.S. patent application Ser. No. 95,782, which issued as U.S. Pat. No. 4,320,507 on Mar. 16, 1982 or European Patent Application No. 79302622.0, tests of various functions can be performed as hereinafter described. However, the above-mentioned prior art system cannot perform a test for detecting poor insulation between the word lines. Such poor insulation between adjacent word lines would cause write-in errors when the user writes a pattern of desired information into the field programmable device, even after the manufacturer has written a test pattern of information into the test memory cells without any trouble.